Electric fuse having magnetic arcquenching action



E. SALZER Sept. 27, 1966 ELECTRIC FUSE HAVING MAGNETIC ARC-QUENGHING ACTION 4 Sheets-Sheet 1 Filed April 50, V 1965 FlG.l

MM KW E. SALZER 3,275,771

ELECTRIC FUSE HAVING MAGNETIC ARC-QUENCHING ACTION Sept. 27, 1966 4 Sheets-Sheet 2 Filed April 50, 1965 INVENTOR.

E. SALZER Sept. 27, 1966 ELECTRIC FUSE HAVING MAGNETIC ARC-QUENCHING ACTION Fild April 30, 1965 4 Sheets-Sheet 5 Sept. 27, 1966 E. SALZER 3,275,771

ELECTRIC FUSE HAVING MAGNETIC ARC-QUENCHING ACTION Filed April 30, 1965 4 Sheets-Sheet. 4

Venting through openings in outer sandwich plates .k 5% 1 b Venting through it, gaps between plates Win '0 i "II Fl G. 10

INVENTOR.

MW KW United States Patent "ice 3,275,771 ELECTRIC FUSE HAVING MAGNETIC ARC- QUENCHING ACTION Erwin Salzer, Waban, Mass., assignor to The Chase- Shawmut Company, Newburyport, Mass. Filed Apr. 30, 1965, Ser. No. 452,051 12 Claims. (Cl. 200-120) This invention relates to electric fuses, and more particularly to current-limiting fuses.

The rate at which the arc-current decreases following initiation of an are at a point of break depends upon the effectiveness of the heat absorbing action of the pulverulent arc-quenching filler which surrounds the point of break. Where the current density at a point of break is relatively high, the cooling action of the arc-quenching filler may not be sufficiently high to force the current down to Zero as rapidly as desired, or required. In ourrent-limiting fuses quartz sand is generally used as arcquenching filler. At a point of break where the density of the arc current is particularly high the quartz sand surrounding the point of break fuses into a dense fulgurite. Such a fulgurite precludes, or limits, the venting of hot products of arcing from the point of break to other points inside of the fuse tube. Therefore the potential heat absorbing capacity of the arc-quenching filler at points remote from the point of break is not well utilized, or not utilized at all, whereas the arc-quenching filler immediately adjacent to the point of break is overheated. Quartz is a semiconductor which, when incandescent, is a relatively good conductor of electricity. A dense hot fulgurite surrounding the arc path at a point of break is, therefore, in effect a shunt across the are which may continue to carry current after extinction of the latter.

Difficulties of the above described nature can be avoided, or limited by sandwiching the necks or portions of reduced cross-section of fuse links between a pair of plates evolving gases under the heat of electric arcs. Such structures are disclosed and claimed in US. Patent 2,866- 038 to Frederick J. Kozacka; Dec. 23, 1958; Current- Limiting Fuses With Increased Interrupting Capacity; US. Patent 2,892,061 to Frederick J. Kozacka; June 23, 1959; Fuses With Fulgurite Suppressing Means; and US. Patent 3,113,195 to Frederick J. Kozacka, Dec. 3, 1963; Fuse Structures for Elevated Circuit Voltages, all assigned to the same assignee as the present invention.

The above patents make it evident that various arrangements of neck-sandwiching plates on fuse links, and various ways of securing such plates to fuse links, have wide ly varying results. All such arrangements have, however, common features. Wherever the neck, or point of break, of a fuse link is sandwiched between a pair of gas evolving plates, the hot products of arcing are diluted and cooled during their travel to the edges of the sandwiching plates where they meet with the pulverulent arcquenching filler. The longer the hot products of arcing dwell in the narrow space between the neck-sandwiching plates, the more effective the cooling action and diluting action of the latter. Therefore, the size of the neck-sandwiching plates is an important factor controlling their effectiveness. Space limitation may stand in the way of imparting to the neck-sandwiching plates such dimensions as may be necessary, or desirable, for electrical reasons, i.e. as may be necessary, or desirable, to achieve the required degree of cooling and diluting before the products of arcing are allowed to spill into the surrounding pulverulent arc-quenching filler.

It is, therefore, one object of this invention to provide electric fuses which are not subject to the aforementioned drawbacks, or limitations.

Under major fault-current conditions are initiation at a neck, or point of reduced cross-section, ofa fuse link 3,275,771 Patented Sept. 27, 1966 is a phenomenon in the nature of an explosion. When an arc is kindled there is a high difference in pressure inside of the neck-cover-plate-sandwich and the space outside of the neck-cover-plate-sandwich. Therefore, the flow of products of arcing from the neck to the edges of the neck-sandwiching plates is relatively rapid, and their dwelling time in the space between these plates relatively short. In other words, the arc and the envelope thereof tend to rapidly escape from the interplate region where they are subjected to an intense cooling and diluting action.

It is, therefore, another object of this invention to provide electric fuses wherein the escape of the arc, and the hot products of arcing, from the space between the neck-sandwiching plates is delayed and wherein the arc is compelled to stay in intimate contact with gas evolving means, resulting in an intense diffusion of relatively cold gases into the arc stream.

Another related object of the invention is to provide electric fuses wherein electrodynamic forces prevailing between parallel current paths formed by juxtaposed breaks cause an intimate engagement of the arcs at the breaks with a gas evolving insulating barrier arranged between the breaks.

Another object of the invention is to provide electric fuses wherein the thermal duty, or cooling duty, of the pulverulent arc-quenching filler is spread relatively evenly throughout the mass of the filler rather than highly concentrated at relatively few points.

Still another object of the invention is to provide improved electric fuses including neck-sandwiching plates of gas evolving insulating materials.

A further object of the invention is to provide electric fuses whose link structure has a high degree of dimensional stability and can safely be provided with portions of drastically reduced cross-section.

A further object of this invention is to provide improved electric fuses having insulating plate means which serve the dual purpose of supporting the fuse links and of evolving highly effective streams of arc-extinguishing gases.

Other objects and advantages of the invention will become apparent as this specification proceeds, and the features of novelty which characterize the invention will be pointed out with particularity in the appended claims forming part of this specification.

For a better understanding of the invention reference may be had to the accompanying drawing in which:

FIG. 1 is a longitudinal section of a fuse structure embodying the present invention taken along 11 of FIG. 2;

FIG. 2 is a longitudinal section of the fuse structure shown in FIG. 1 taken along 2-2 of FIG. 1;

FIG. 3 is a cross-section of the fuse structure of FIGS. 1 and '2 taken along 33 of FIG. 1 and drawn on a larger scale than FIGS. 1 and 2;

FIG. 4 is a plan view of a blade-contact and fuse link assembly embodying the present invention;

FIG. 4a is a plan view of a modification of the structure shown in FIG. 4;

FIG. 5 is a section taken along 55 of FIG. 4;

FIG. 6 is a plan view of another blade contact and fuse link assembly embodying the present invention;

FIG. 7 is a section taken along 7-7 of FIG. 6;

FIG. 8 is a diagrammatic cross-section of a fuse embodying this invention similar to FIG. 3 but including an indication of the spatial distribution of the thermal duty of the arc-quenching filler and an indication of the positioning of the various fulgurites formed incident to blowing of the fuse;

FIG. 9 refers to another embodiment .of the invention and is a section along 99 of FIG. 10; and

FIG. 10 is a section along 10-10 of FIG. 9.

Referring now to the drawings, and more particularly to FIGS. 1-3 thereof, numeral 1 has been applied to indicate a housing of insulating material as, for instance, a melamine-glass-clotl1 laminate. Terminal elements in form of blade contacts 2 are arranged at the axially outer ends of housing 1. Each blade contact 2 projects from the outside of housing 1 through one of a pair of terminal caps 3 into the inside of housing 1. Washers 4 preferably of an inorganic fibrous material such as asbestos are interposed between the rims of housing 1 and caps 3. Resilient hollow spring pins 5 project transversely through housing 1 and blade contacts 2. Drive screws 6 or similar expansion means for the ends of spring pins 5 are driven into the latter to preclude any displacement of pins 5 in a direction longitudinally thereof. For further particulars in regard to spring pins 5 and the function thereof reference may be had to US. Patent 3,007,020 to Frederick J. Kozacka, Oct. 31, 1961, Fuse Structures, disclosing in detail an assembly of this nature. A fuse link support plate 7 of insulating material evolving gas when exposed to the heat of an electric arc is arranged inside of housing 1 within the space bounded by the axially inner edges of knife blade contacts 2. Fuse link support plate 7 may be made of an organic insulating material as, for instance, a melamine-glass-cloth laminate, if precautions are taken not to subject the link support plate 7 to excessive temperatures, as will be explained below more in detail. Reference character 8 has been applied to indicate a pair of ribbon fuse links having a spacing equal to the thickess of fuse link support plate 7. Fuse links 8 engage physically with the juxtaposed sides or surfaces thereof fuse link support plate 7 and are supported by the latter. Each fuse link 8 has two bends of 90 degrees adjacent each of the ends thereof, and the ends of fuse links 8 overlap the axially inner ends of knife blade contacts 2. The ends of fuse links 8 which overlap the axially inner ends of knife blade contacts 2 are conductively connected to knife blade contacts 2 as, for instance, by brazing joints. Each fuse link 8 is provided with three transverse lines 8a of circular perforations forming three serially related points of reduced crosssection. The perforations formed in fuse link 8 above fuse link support plate 7 and the perforations formed in fuse link 8 below fuse link support plate 7 (FIG. 1) are arranged in registry. When fuse links 8 carry relatively high fault currents as, for instance, short-ciruit currents, fuse links 8 fuse at their points of reduced cross-section and points of maximal current density, and electric arcs are then kindled at these points. The magnetic fields resulting from these arcs draw these arcs into intimate engagement with link support plate 7. The magnetic or electrodynamic attraction of registering arcs increases inversely to their spacing. Therefore it may be desirable to make fuse link support plate 7 relatively thin, i.e. thinner than knife blade contacts 2. If plate 7 is made of a dimensionally stable material such as, for instance, the aforementioned melamine-glass-cloth the thickness thereof may be reduced as necessary to intensify arc attraction, without sacrificing mechanical strength requirements. If desired, the width of link support plate 7 may substantially exceed the width of fuse links 8. The width of link support plate 7 may even be equal to the inner diameter of casing 11, and plate 7 may be press-fitted into and supported by the latter. A pair of cover plates 9 of an insulating material evolving gas when exposed to the heat of an electric arc sandwiches the aforementioned points of reduced cross-section, or the perforated areas, of fuse links 8 situated on opposite sides-of fuse link support 7. Eyelets or rivets 10 project transversely through both plates 9, both fuse links 8 and fuse link support plate 7 and thus integrate these five layers into a unitary structure. Each cover plate 9 is provided with a rectangular cut-out or window 9a at the center region thereof exposing the transverse center line 8a of perforations. Each fuse link 8 supports a link-severing overlay 11 of tin adjacent the transverse center line 811 of perforations thereof. The fuse links 8 proper are made of a metal combining low specific resistance and small fusing ft dt values, preferably silver or copper. The overlays 11 of tin may be replaced by overlays of other metals having a substantially lower fusing point than silver and copper, and capable of rapidly severing the base metal by a metallurgical reaction, i.e. a diffusion process, when their fusing point is reached. Overlays 11 may be arranged on fuse links 8 in various different fashions. A preferred arrangement for overlays 11 is disclosed in US. Patent 2,988,620 to Frederick J. Kozacka, June 13, 1961, Time-Lag Fuses, assigned to the same assignee as the present invention. Housing 1 is filled with a pulverulent arc quenching filler 12, preferably quartz-sand.

On occurrence of overload currents of inadmissible duration fusion of overlays 11 severs both fuse links 8 at the center region thereof. This results in interruption of V the overloaded circuit. As long as the current carried by the fuse is in the load and overload range, i.e. is not of short-circuit current proportions, the center regions of the fuse links 8 are the hottest points of the fuse structure. The temperature at the hottest points of the fuse links 8 is less than the fusing point of overlays 1 1 and the temperature at the regions of the axially outer lines 8a of transverse perforations is "less than that of the hottest points of the fuse links 8. Plates 9 of organic insulating material are not subject to the temperature prevailing at the hottest points of the fuse links because of the presence of cut-outs or windows 9a. As a result of the circumstances set forth above, plate 7 as well as plates 9 may be made of an organic insulating material, such as a synthetic resin-glass-cloth laminate, without being subject to significant thermal degradation.

Plate 7 may show evidence of ageing, but the current density at the hottest points of links 8 may be kept sufficiently low to avoid serious harm to plate 7.

On occurrence of currents of short-circuit current proportions fuse links 8 fuse along their transverse lines 8a of perforations. Hence parallel registering arcs are formed at each point of break above and below plate 7. These registering arcs are attracted to each other by electrodynamic action. In FIG. 3 the forces of electrodynamic attraction of the registering arcs have been indicated by vectors to which reference character E has been applied. As a result of the intimate engagement under the action of electrodynamic forces between registering arcs and link supporting plate 7, the latter evolves high velocity jets of gas B tending to flow in a direction opposite to the aforementioned vectors E In the center regions of fuse links 8 the jets of gas B are allowed to flow through cut-outs or windows 9a directly into the pulverulent arc-quenching filler 12. In the axially outer regions of fuse links 8 where the transverse lines 8a of perforations are located plates 9 evolve high velocity jets of gas under the action of the arcs located at these points. The direction of these jets of gas tends to be opposite to the direction of the jets B In other words, while the arcs are firmly held in position, or restrained, by the action of electrodynamic forces and thus unable to escape, they are being blasted from opposite sides by streams of un-ionized and relatively cool gases. This results in an optimal interdiifusion of hot and cold gases and cooling of the former. When the products of arcing reach the longitudinal edges of plates 9-which may be recessed from the longitudinal edges of link supporting plate 7 (see FIG. 3) the-se gases may have been cooled down to such an extent as to safely preclude any significant fusion, or sintering, of the particles of quartz-sand 12 into which they flow. Whenever there is significant sintering of particles of quartz-sand this inhibits, or precludes, the free flow of products of arcing from the arcing zone into the surrounding body of quartz-sand and, therefore, keeps the latter from participating to the desired extent in the process of.

cooling and are extinction. However, the structure of FIGS. 13 is not subject to this limitation.

It will be apparent that are extinction at the center of the fuse links 8 differs from the process of arc extinction adjacent the axially outer ends of fuse links 8. At the center of fuse links 8 the captive arcs are subjected to substantially unidirectional jets of gas which impinge immediately upon the quartz-filler 12. Adjacent the outer ends of fuse links 8 the captive arcs are subjected to cross-jets of relatively cool gases, and the products of arcing are not allowed to impinge upon the arc-quenching quartz-tiller 12 until after they have dwelled for some time between plates 7 and 9, and have been further cooled down during that time.

It is possible to design fuse structures wherein all captive arcs are allowed to freely vent into the pulverulent filler such as is the case in regard to the arcs formed at the centers of fuse links 8 of the structure of FIGS. 1-3. On the other hand, it is also possible to design fuse structures wherein all captive arcs are inhibited from freely venting into the pulverulent filler such as is the case in regard to the arcs formed adjacent the axially outer ends of the fuse links 8 of the structure of FIGS. 1-'3.

As a general rule it is desirable to combine both modes of arc extinction because this combination affords a higher flexibility as will be shown below more in detailand is conducive to maintaining a relatively stable arc voltage during the entire arcing time.

The force F between parallel conductors carrying the same current i and whose length is Al and whose spacing is r is given by the equation wherein [.l. is the magnetic permeability of the medium surrounding the two conductors. Sinceas is apparent from the above equationthe forces of attraction of the arcs formed at points of reduced cross-section, or necks, of :fuse links 8 increase with the second power of the current, these forces are very large on occurrence of fault currents of short-circuit current magnitude, and tend to mechanically break the necks before the faulted circuit has supplied the aggregate fusing and vaporization integral i -dt values theoretically required for formation of a break by fusion and vaporization of a neck, or point of reduced cross-section, of a fuse link. The way in which the necks of the links of a fuse structure embodying this invention are severed and breaks are formed at the points where the necks were located depends on a number of parameters. The following steps are significant in the process of severing the fuse links and forming breaks at the point-s where the necks or points of reduced cross-section are located.

The intense electrodynamic forces prevailing between registering necks on occurrence of major fault currents tend to destroy or sever the necks mechanically. The registering arcs which are then kindled at each point Where a neck or point of reduced cross-section had been located are held captive in position by their mutual electrodynamic attraction. While the gases evolved from plate 7, or from plates 7 and 9, are not capable of diffusing instantly into the core region of the arcs, they penetrate into the arc envelopes and dilute the latter and tend to increase the ionization gradient at the arcing zones. This results in an instant decrease of the arc current and consequently in a decrease of the mutual attraction of the registering arcs to both sides of plate 7. As the forces of attraction between the registering arcs decrease the gases generated from plate 7, or from plates 7 and 9, sweep the low current arcs away from the points where they had been captive and into a cooler region. This results in final arc extinction. The entire process of circuit interruption lasts considerably less than a quarter of a cycle of a current wave of 60 c.p.s.

Referring now to FIGS. 4 and 5, these figures show a fuse structure intended to be mounted in a casing as shown in FIGS. 1 and 2. In FIGS. 4 and 5 the casing has been deleted. Reference character 2 has been applied to indicate a pair of blade contacts forming a gap between the axially inner ends thereof. This gap is filled by an elongated member or plate 7' of an organic insulating material, i.e. an insulating material evolving gas when subjected to the heat of electric arcs. Plate 7 is sandwiched between a pair of ribbon fuse links 8 each having five transverse lines 8a of circular perforations. These perforations define serially related points of reduced crosssection, or necks, where the current density is highest. The axially outer ends of fuse links 8' are bent twice at an angle of degrees and form tabs sandwichin'g the axially inner ends of blade contacts 2. The ends of links 8 are conductively connected to blade contacts 2' by brazing joints, or like current-carrying connecting means. The width W of plate 7' of gas evolving insulating material slightly exceeds the width w of fuse links 8. Plates 9 of an insulating material evolving gases when subjected to the heat of electric arcs sandwich fuse links 8' and their supporting plate 7. Rivets, eyelets, or like fasteners 10' project through the entire stack formed by parts 7, 8' and 9, thus integrating this stack into a unitary structure. Fuse links 7 are of silver, or copper, i.e. metals having a relatively high conductivity, a relatively high fusing point and a relatively low fusing fi -dt. Each fuse link 7' supports adjacent the transverse center line So of perforations an overlay 11' of a relatively low fusing point linksevering metal as, for instance, tin. Plates 9 are provided with cut-outs or windows 9a which expose the transverse center lines 8a of perforations and the adjacent regions of links 8 including tin overlays 11 to the immediate engagement with pulverulent arc-quenching filler 12' in which the stack formed of parts 7, 8', 9 is submersed. Plates 9' are further provided with rectangular cut-outs or windows 9b. The latter are arranged above and below non-perforated areas of fuse links 8 or, in other words, cut-outs or windows 9b are displaced with respect to the transverse lines 8a of perforations in a direction longitudinally of fuse links 8. The fasteners 10' are arranged in registry with the transverse lines 8a of perforations, or are located along the transverse lines defined by the centers of the perforations. Therefore fasteners 10' tend to impede the flow of products of arcing from the points of break defined by the constituent perforations of each transverse line 8a in a direction trans verse to links 8 into the arc-quenching filler 12'. To be more specific, the flow resistance of the path along which the products of arcing may escape from the points of break is less in a direction longitudinally of links 8 than in transverse directionv Consequently the products of arcing flow in the fashion indicated by arrows S and escape through cut-outs 9b and 9a into filler 12'. The arcs formed at each of the many points of break are immobilized by electrodynamic attraction, and while so immobilized subjected to intense jets of relatively cool gas evolving from plate 7 and from plates 7' and 9. The products of arcing formed at the necks defined by the transverse center lines So of perforations of links 8' are directly released into the pulverulent arc-quenching filler 12. The products of arcing formed at the necks defined by the four axially outer transverse lines So of perforations flow along planes parallel to fuse links 8 situated between plates 7 and 9' before being released into the arc-quenching filler 12. During the travel of the products of arcing in a direction longitudinally of fuse links 8' along the aforementioned planes the products of arcing are subjected to a dual cooling action. (1) Since the temperature of the areas of fuse links 8- along which the products of arcing escape to cut-outs 9b is far less than that of the products of arcing, the products of arcing are cooled, and fuse links 8 are heated, in the increments of time during which the products of arcing travel to filler 12.

The abstraction of heat by fuse links 8' is desirable because it reduces the energy which must be supplied by the faulted circuit to achieve a predetermined, or desired, degree of back-burning. (2) The other action which tends to cool the products of arcing during the travel thereof in a direction longitudinally of links 8' is the interdiffusion of relatively cool gas evolved from plates 7' and 9' Thus the products of arcing are relatively cool at the time they reach the arc-quenching filler 12'. Moreover, the area where the products of arcing initially meet with the arcquenching filler is relatively large. Consequently, the density of the fulgurites formed at these points is relatively small, and such low density fulgurites do not form any serious obstruction to the flow of products of arcing through the fulgurites to relatively remote regions of the arc-quenching filler. This, in turn, results in a relatively uniform participation in the interrupting process of the entire arc-quenching filler by which stack 7, 8', 9' is surrounded. As mentioned above, the formation of dense fulgurites significantly obstructing the relatively free flow of products of arcing away from the arcing zones to regions remote from the arcing zones keeps the arc-quenching filler at points remote from the arcing zones from participating, or fully participating, in the interrupting process.

The above refers to the performance of the structure of FIGS. 4 and 5 under short-circuit current conditions. On occurrence of overloads overlays 11' sever fuse links 8' adjacent the center thereof, resulting in interruption of the overloaded circuit.

In drawing FIG. 4a the requisite tubular casing has been omitted in the same fashion as in drawing FIGS. 4 and 5. The structure of FIG. 4a comprises blade contacts 2", a link supporting plate 7", and a pair of fuse links 8" defining a plurality of narrowly spaced points of reduced crosssection and is, as far as these parts and the arrangement thereof are concerned, identical to what is shown in FIGS. 4 and 5. The structure of FIG. 4a differs from the structure of FIGS. 4 and 5 in regard to the geometryof the two outer insulating plates which sandwich the fuse links and the centrally located fuse link supporting plate 7". The structure of FIG. 4a differs also from the structure of FIGS. 4 and 5 in regard to the arrangement of the fasteners which integrate the layers of the structure into a structural unit. According to FIG. 411 each of two superimposed fuse links 8" is provided with five transverse lines 8a ofcircular perforations. Plates 9" covering fuse links 8" are provided with three rectangular windows, or cut-outs 9a, each exposing three out of five transverse lines 8a" of circular perforations to the immediate action of a pulverulent arc-quenching filler l2" surrounding the structure of FIG. 4a. The remaining two transverse lines 8a" of circular perforations are separated from immediate action of the pulverulent arc-quenching filler 12" surrounding the structure of FIG. 4a by the portions of plates 9" which are not provided with cutouts or windows. The fasteners are arranged in such a fashion that they tend to obstruct venting from the regions of two of the transverse lines 8a" of perforations in a direction longitudinally of fuse links 8". The arc products fonmed in these regions tend to escape in transverse directions, as indicated by the arrows S. The arcvoltage generated at the points of break exposed to the immediate action of the arc-quenching filler is in the shape of a spike of relatively short duration rising rapidly and decaying rapidly. The are voltage generated at the points of break not exposed to the immediate action of the pulverulent arc-quenching filler '12" is of relatively smaller magnitude and relatively longer duration than the first mentioned are voltage. Therefore the aggregate arc voltage is relatively stable and of relatively long duration. In the structure of FIG. 4a parallel narrowly spaced arcs are held captive by electrodynamic action and-while being held captive-are exposed to the action of intense 8 arc-generated blasts of gas, as more fully described in connection with FIGS. 1-5.

The structure of FIGS. 6 and 7 combines the features of electromagnetic arc-attraction against a gas evolving plate with that of arc-voltage stabilization as occurs in the structure of FIG. 4. The former is, however, easier and less expensive to manufacture since it dispenses with one of the link covering plates.

Referring now to FIGS. 6 and 7, the structure shown therein comprises a pair of spaced knife blade contacts 2' conductively interconnected by a pair of ribbon fuse links 8" having five transverse lines 8a" of circular perforations. The perforations and the points of minimum cross-sectional area of both fuse links 8" are arranged in registry, resulting in an attraction of the current-paths at the points of minimum cross-sectional area toward each other and against the gas-evolving insulating linksupport 7". The upper fuse link 8' is provided with a gas-evolving cover plate 9 having a cut-out portion or window 9a. This window 911" exposes a link-severing overlay 11" of tin, or a like low fusing point metal. The lower fuse link 9' is not provided with a gas-evolving cover plate, but is directly exposed to the action of a pulverulent arc-quenching filler 12" surrounding all the parts situated within the gap bounded by contacts 2". The lower fuse link 9 is provided with a link-severing overlay 11 of tin or a like low fusing point metal arranged in registry with the overlay 11 on upper fuse link 9". The stack comprising layers or parts 9, 7", 8" is held together by rivets or eyelets 10" projecting through the constituent parts of the aforementioned stack. The rivets or eyelets 10 are positioned in such a fashion that the minimum resistance to the flow of products of arcing away from the points of narrowest cross-section of upper link 8" is substantially in a direction transversely to the direction in which links 9 extend. In FIG. 6 the direction or paths in which the products of arcing escape from the points of narrowest cross-section of upper link 8" has been indicated by arrows to which reference character 8" has been applied.

On occurrence of overloads 0f inadmissible duration links 8" are severed by alloy-formation between the base metal and the overlay metal of these two links. On occurrence of short-circuit currents links 8 fuse virtually simultaneously at all their points of reduced cross-section and the resulting arcs are attracted by electromagnetic or electrodynamic action against link supporting plate 7". This results in the evolution of opposite gas blasts from plate 7". The blast of gas generated at the lower surface of plate 7" flows immediately into the surroundmg arc-quenching filler, thus being rapidly cooled. Therefore the dielectric recovery of the gaps formed by the lower fuse link 8'" is relatively rapid, i.e. more rapid than the dielectric recovery of the gaps formed by the upper fuse link 8". The rise of the arc-voltage at the gaps formed by upper fuse link 8" is somewhat slower than that at the gaps formed by the lower fuse link 8', this difference being due to the fact that the paths 3" of the products of arcing before reaching the arc-quenching filler 12 is longer for the upper fuse link 8 than the lower fuse link 8". Fusion of the lower link 8" results in a fulgurite which may be coextensive with lower link 8". No such fulgurite forms where the upper link 8" is located because of the spatial separation of the points of break and the arc-quenching filler 12" and because ventmg of the products of arcing is limited to specific points along the longitudinal edges of plates 7' and 8".

The faster generation of arc-voltage at the lower fuse link 8" results in a current transfer to the upper fuse link 8". The arcs kindled at the lower fuse link 8" are extinguished before the arcs kindled at the upper fuse link 8" are extinguished. Thus the relatively long fulgurite formed adjacent the location of the lower fuse link 8 has a short interval of time to cool down before the arc path taking the place of the upper fuse link 8 loses its conductivity. This interval of time, as short as it is, is sufficiently long to some cooling of the fulgurite which is formed adjacent the location of the lower fuse link 8", thus rendering this fulgurite harmless. As will be apparent from the foregoing the presence of insulating plate 9" precludes the formation of a relatively long or continuous ful-gurite adjacent the location of the upper fuse link 8".

Referring now to FIGS. 9 and 10 reference character 1" has been applied to indicate a casing of insulating material closed at both ends by terminals caps 3". Casing 1" houses a strip of copper-clad insulating material comprising an inner layer of gas evolving insulating material, e.g. melamine-glass-cloth, and a pair of outer layers 8 of sheet copper bonded to the inner layer 20 Each copper layer 8" is severed at five serially related points by a transverse groove 21. Grooves 21 in upper layer 8"" and in lower layer 8" are arranged in registry. Reference numeral 22 has been applied to indicate two pairs of parallel tin-plated silver wires arranged at right angles to grooves 21, one pair being soldered to the upper copper layer 8" and the other pair being soldered to the lower copper layer 8". The upper silver Wires 22 and the lower silver wires 22 are arranged in registry. Therefore the points of upper and lower wires 22 arranged in registry which bridge the grooves 21 in upper and lower copper layers 8" are mutually attracted by electrodynamic action. The same applies in respect to the arcs which take the place of these points upon fusion of wires 22 at these points. The above arcs are, therefore, drawn into grooves 21 and into intimate engagement with gasevolving insulating layer 20. As a result of this engagement the arcs drawn into grooves 21 and held captive therein are subject to intense cross-blasts of gas. The ionized gases are allowed to escape through grooves 21 into an arc-quenching, body of quartz sand 23 occupying the volume of casing 1"" not occupied by other parts arranged therein. Wires 22 are sandwiched between copper layers 8"" and cover plates 24 of gas evolving insulating material, e.g. a melamine-glass-cloth laminate. Cover plates 24 are secured to copper clad strip 8", 20, 8" by means of eyelets 25 projecting transversely through said strip and both cover plates 24. Eyelets 25 are arranged along the center line of strip 8"", 20, 8" and cover plates 24 which cover most or the entire length of grooves 21. Thus each groove 21 defines a closed transverse duct having two orifices for discharging hot products of arcing into the arc-quenching body 23 of quartz sand. If the pressure inside of grooves 21 becomes excessive, plates 24 are bent away from copper layers 8"", thus significantly widening the discharge area of grooves 21. In other words, cover plates 24 operate as safety valves against dangerous pressure build-up in grooves 21. Each cap 3"" is provided with a rectangular slot through which the axially outer ends of copper clad strip 8", 20, 8"" project from the inside of easing 1"" to the outside thereof. Each cap 3"" defines in the center thereof a circular dish-shaped recess filled with a relatively thin layer 26 of solidified solder. solder establish current paths of low resistance between copper layers 8" and caps 3".

In the structure of FIGS. 9 and 10 cover plates 24 may be replaced by extrusions of suitable synthetic resins, e.g. epoxy resins, each covering one of the current-carrying wires 22. Such extrusions encapsulate the points of reduced cross-section of the current-paths of the structure of FIGS. 9 and 10, i.e. they envelope wires 22 at all the points where the same span grooves 21 in copper layers 8.

It will be understood that I have illustrated and described herein preferred embodiments of my invention, and that various alterations may be made therein without departing from the spirit and scope of the appended claims.

These layers or pools of I claim as my invention:

1. An electric fuse comprising in combination:

(a) a substantially tubular housing of insulating material;

(b) a pair of terminal elements arranged at the axially outer ends of said housing;

(c) an elongated flat plate .of an organic insulating material evolving gases under the heat of electric arcs metal clad on both surfaces thereof and forming a pair of narrowly spaced parallel current paths each defining a plurality of registering points of reduced cross-section arranged inside said housing;

(d) means for conductively connecting the ends of said member to said pair of terminal elements; and

(e) a pulverulent arc-quenching filler inside said housing limited to space radially outward from said plate and said metal cladding thereof.

2. An electric fuse as specified in claim 1 wherein at least a portion of said registering points of reduced crosssection is separated from said arc-quenching filler by insulating barrier means evolving gases under the heat of electric arcs.

3. An electric fuse comprising in combination:

(a) a substantially tubular housing of insulating material;

(b) a pair of terminal elements arranged at the axially outer ends of said housing;

(c) a planar fuse link support plate of an organic insulating material evolving gas when exposed to the heat of an electric arc arranged inside said housing;

((1) a pair of ribbon fuse links having a spacing equal to the thickness of said fuse link support plate physically engaging said fuse link support plate with juxtaposed sides thereof and being supported by said fuse link support plate conductively interconnecting said pair of terminal elements, said pair of fuse links having points of reduced cross-section arranged in registry on opposite sides of said fuse link support plate to magnetically draw the arcs initiated at said points of reduced cross-section against, and into intimate engagement with, said fuse link support plate; and

(e) a pulverulent arc-quenching filler inside said housing limited to space radially outward from said pair of fuse links and substantially excluded from the space between said pair of fuse links.

4. An electric fuse comprising in combination:

(a) a substantially tubular housing of insulating material;

(b) a pair of terminal caps arranged at the axially outer ends of said housing and closing said housing;

(0) a pair of blade contacts having a predetermined thickness extending in a direction longitudinally of said housing each projecting from the outside of said housing through one of said pair of caps into the inside of said housing;

(d) a flat fuse link support of a gas evolving insulating material having a smaller thickness than said predetermined thickness of said pair of blade contacts arranged within said housing inside of the gap defined between the axially inner ends of said pair of blade contacts;

(e) a pulverulent arc-quenching filler inside of said housing enveloping said fuse link support; and

(f) a pair of ribbon fuse links having a spacing equal to the thickness of said fuse link support physically engaging said fuse link support with juxtaposed sides thereof and being supported by said fuse link support conductively interconnecting said pair of blade contacts, said pair of fuse links having points of reduced cross-section arranged in registry on opposite sides of said fuse link support to magnetically draw arcs initiated at said points of reduced cross-section against and into intimate engagement with said fuse link support and away from said arc-quenching filler.

5. An electric fuse comprising in combination:

(a) a substantially tubular housing of insulating material;

(b) a pair of terminal elements arranged at the axially outer ends of said housing;

(c) a flat fuse link support plate of an insulating material evolving gas when exposed to the heat of :an electric are arranged inside said housing;

(d) a pair of ribbon fuse links having a spacing equal to the thickness of said fuse link support plate physically engaging said fuse link support plate with juxtaposed sides thereof and being supported by said fuse link support plate conductively interconnecting said pair of terminal elements, said pair of fuse links having points of reduced cross-section arranged in registry on opposite sides of said fuse link support plate to magnetically draw the arcs initiated at said points of reduced cross-section against and into intimate engagement with said fuse link support plate;

(e) plate means of an insulating material evolving gas when exposed to the heat of electric arcs sandwiching at least a portion of said points of reduced crosssection of said pair of fuse links and sandwiching said :fuse link support; and

(f) a pulverulent arc-quenching filler enveloping said fuse link support and said pair of fuse links.

6. An electric fuse comprising in combination:

(a) a substantially tubular housing of insulating material;

(b) terminal elements arranged at the axially outer ends of said housing;

() a pulverulent arc-quenching filler inside said hous- (d) a fuse link support of an insulating material evolving gas when exposed to the heat of electric arcs arranged inside said housing and submersed in said filler;

(e) a pair of ribbon fuse links conductively interconnecting said pair of terminal elements having a spacing equal to the thickness of said fuse link support being supported by said fuse link support and physically engaging juxtaposed sides of said fuse link support, each of said pair of fuse links having a plurality of points of reduced cross-section arranged in registry on opposite sides of said fuse link support; and

( f) a baflle plate of insulating material evolving gas when exposed to the heat of electric arcs extending over at least a portion of one of said pairs of fuse links, said bafiie plate having cut-outs allowing direct venting from a portion of said plurality of points of reduced cross-section of said one of said pair of fuse links into said arc-quenching filler, and said bafile plate covering another portion of said points of reduced cross-section of said one of said pair of fuse links and having additional cut-outs spaced from said another portion of said points of reduced crosssection of said one of said pair of fuse links in a direction longitudinally of said one of said pair of fuse links and allowing venting of products of arcing from said another portion of said points of reduced cross-section of said one of said pair of fuse links into said arc-quenching filler over a relatively extended venting path.

7. An electric fuse comprising in combination:

(a) a substantially tubular housing of insulating material;

(b) terminal elements arranged at the axially outer ends of said housing;

(c) a pulverulent arc-quenching filler inside said hous- (d) a fuse link support of an insulating material evolving gas when exposed to the heat of electric arcs arranged inside said housing and submersed in said filler;

(e) a pair of ribbon fuse links conductively interconnecting said pair of terminal elements having a spacing equal to the thickness of said fuse link support and being supported by said fuse link support, said pair of fuse links having points of reduced crosssection arranged in registery on opposite sides of said fuse link support, a portion of said points of reduced cross-section being in immediate physical engagement with said arc-quenching filler; and

(f) baflle means of insulating material evolving gas when exposed to the heat of electric arcs covering an. other portion of said points of reduced cross-section and extending the path of venting from said another portion of points of reduced cross-section into said arc-quenching filler.

8. An electric fuse comprising in combination:

(a) a substantially tubular housing of insulating material;

(b) terminal elements arranged at the axially outer ends of said housing;

(c) a pulverulent arc-quenching filler inside said hous- (d) a fuse link support of an insulating material evo1ving gas when exposed to the heat of electric arcs arranged inside said housing and submersed in said filler;

(e) a pair of ribbon fuse links interconnecting said pair of terminal elements having a spacing equal to the thickness of said fuse link support and being supported by said fuse link support, each of said pair of fuse links having a plurality of points of reduced cross-section arranged in registry on opposite sides of said fuse link support, said plurality of points of reduced cross-section of one of said pair of fuse links being in immediate physical engagement with said arc-quenching filler; and

( f) a baffle plate of insulating material evolving gas when exposed to the heat of electric arcs covering said plurality of points of reduced cross-section of the other of said pair of fuse links and extending the path of venting from said plurality of points of reduced cross-section of said other of said pair of fuse links into said arc-quenching filler.

9. An electric fuse comprising in combination:

(a) a substantially tubular housing of insulating material;

(b) a pair of terminal elements arranged at the axially outer ends of said housing;

(c) an elongated member of an insulating material evolving gas when exposed to the heat of electric arcs arranged inside said housing;

(d) a pair of metal layers each bonded to one side of said member and each conductively connected with one end thereof to one of said pair of terminal elements, said pair of metal layers being interrupted by transverse registering grooves exposing transverse registering strips of opposite surfaces of said member; and

(e) a pair of fusible wires extending in a direction longitudinally of said housing bridging said transverse registering grooves and each engaging one of said pair of metal layers.

10. An electric fuse as specified in claim 9 wherein said pair of fusible wires is of a metal having a relatively high conductivity and a relatively high fusing point and is coated throughout the entire length thereof with relatively low fusing point solder and wherein each of said pair of fusible wires is solder-connected to one of said pair of metal layers throughout the entire length thereof.

11. An electric fuse as specified in claim 9 wherein at least one of said fusible wires is covered by gas evolving insulating barrier means at least at points where bridging said grooves interrupting said metal layers.

12. An electric fuse comprising in combination:

(a) a substantially tubular housing of insulating material;

(b) a pair of terminal elements arranged at the axially outer ends of said housing;

(c) a pulverulent arc-quenching filler inside said hous- (d) an elongated member of insulating material evolving gas when exposed to the heat of electric arcs arranged inside of said housing and submersed in said filler;

(e) a pair of copper layers each bonded to one side of said member and each conductively connected with one end thereof to one of said pair of terminal elements, said pair of metal layers being interrupted by transverse registering grooves exposing transverse registering strips of opposite surfaces of said member;

(f) a pair of silver wires extending in a direction longitudinally of said housing, and bridging said trans verse registering grooves interrupting said pair of metal layers, said pair of silver wires being coated throughout the entire length thereof with a relative- 1y low fusing point solder and each of said pair of fusible wires being solder connected throughout the entire length thereof to one of said pair of metal layers; and

(g) overlay means of a synthetic resin covering at least portions of at least one of said pair of silver wires where bridging one of said registering strips.

References Cited by the Examiner BERNARD A. GILHEANY, Primary Examiner.

H. B. GILSON, Assistant Examiner. 

1. AN ELECTRIC FUSE COMPRISING IN COMBINATION: (A) A SUBSTANTIALLY TUBULAR HOUSING OF INSULATING MATERIAL; (B) A PAIR OF TERMINAL ELEMENTS ARRANGED AT THE AXIALLY OUTER ENDS OF SAID HOUSING; (C) AN ELONGATED FLAT PLATE OF AN ORGANIC INSULATING MATERIAL EVOLVING GASES UNDER THE HEAT OF ELECTRIC ARCS METAL CLAD ON BOTH SURFACES THEREOF AND FORMING A PAIR OF NARROWLY SPACED PARALLEL CURRENT PATHS EACH DEFINING A PLURALITY OF REGISTERING POINTS OF REDUCED CROSS-SECTION ARRANGED INSIDE SAID HOUSING; 